Rett syndrome is a neurological disease of early childhood. It is associated with deleterious mutations of the gene encoding methyI-CpG-binding protein 2 (MECP2) but it remains unclear how MECP2-deficiency results in neuronal disease. MECP2 is thought to regulate acetylation, methylation and other post-translational modifications of the core histones, that together with DNA wrapped around them comprise the fundamental structural unit of chromatin and thus regulate gene expression, DNA repair and chromosome segregation. Our central goals are 1) to test the hypothesis that histone hyperacetylation contributes to the Rett syndrome phenotype (Specific Aim 1). We will monitor in developing cerebral cortex of wildtype and mutant mice developmentally regulated changes in H3 and H4 covalent modifications at the site of regulatory sequences of ionotropic glutamate receptor subunit genes, at the beta-globin locus and at methyI-CpG-rich regulatory sequences of the imprinted gene Necdin and of the Xist gene (Specific Aim 2). We will examine in primary cortical cultures if drug-induced changes in histone acetylation affect neuronal growth and survival (Specific Aim 3). Finally, we will study histone methylation in postmortem cerebral cortex of subjects diagnosed with Rett syndrome and confirmed MECP2 mutations, in comparison to subjects diagnosed with autism spectrum disorder and to non-neurological controls. Based on preliminary data, our central hypotheses are: 1) Treatment with drugs that inhibit histone deacetylases (HDACs) will accelerate development of disease in MECP2-deficient mice; 2) HDAC inhibitors impair growth and survival of cultured MECP2-deficient neurons; 3) Histone methylation is dysregulated in cerebral cortex of Rett subjects; 4) developmentally regulated histone acetylation and methylation at defined genomic regions is altered in brain of MECP2 mutant mice. Our experiments include in vivo and vitro studies with HDAC inhibitor and activator drugs, using conditional mutant mice with cre/IoxP mediated Mecp2 excision in cerebral cortex at different stages of development, complemented by a collection of human postmortem tissue. Our experiments will rely on chromatin immunoprecipitation assays, immunoblotting and immunohistochemistry with antibodies against H3 and H4 epitopes defined by site-specific modifications. It is expected that these approaches will provide a clear and comprehensive picture on the developmental regulation of histone modifications in cortical neurons, including potential changes in MECP2-deficient brain.